1. Field of the Invention
The present invention relates to a rotary actuator for rotatably actuating a controlling device, such as a multi-way valve, to an angle corresponding to the magnitude of the input to the actuator.
2. Description of the Prior Art
A known rotary actuator for providing an angle of rotation of less than about 180.degree. has a bipolar motor having a return coil spring connected to the shaft of the motor so that the motor shaft is rotated to a position where a balance is attained between the driving torque produced electro-magnetically in accordance with the input current and the recovery force exerted to the motor shaft by the return spring.
The known rotary actuator, however, involves a problem that, as shown in FIG. 1, the angle of rotation attained with a given magnitude of input current during an increase of the input current is different from that attained with the same magnitude of input current in the course of decrease of the input current. This difference is attributable to various factors such as the friction between the motor shaft and bearings, friction between turns of the coil spring and so forth. Among these various factors, the mutual contact between the adjacent turns of the coil spring and the contact between the coil spring and other component parts are most influential. The influence of these factors is more serious in the case where the rotary actuator is mounted on a vehicle such as an automobile.